init txn thread private data for all database modes
    
    Even though the indexing code explicitly disables the database opening in
    transaction mode, it still uses transactions.  So we have to init the
    thread private data for transactions in all cases.  This doesn't hurt
    anything, just creates a thread private key.  Also, in the indexing code,
    move the dblayer_txn_init until after dblayer_start has been called.
    Reviewed by: nkinder (Thanks!)